There were probably a thought that older and obsolete content could damage website reputation and rankings. Some were also scared of Panda as if having older content on site may invite Google wrath. Please read more details here : http://www.brainpulse.com/articles/seo-sem/outdated-content-removal-and-ranking.php

I think ... Do not delete old content that should update it. Make sure that the new content updates on your website is unique, taking engage the reader, reasonable keyword density, .... above all there together with structured data "dateModified".